1. Field of the Invention
This invention relates to cement concrete-carbon fiber composites which display high tensile and flexural strengths, low electrical resistivity and high electromagnetic interference shielding effectiveness.
2. Description of the Prior Art
The addition of short or continuous inorganic fibers (glass, asbestos, steel, carbon, etc.) increases the tensile and flexural strengths of concrete cements. However, asbestos fibers are carcinogenic, steel fibers tend to rust and glass fibers deteriorate in the highly alkaline environment of cement. Carbon fibers are inert, medically safe, as strong as steel fibers and more chemically stable than glass fibers in an alkaline environment. Moreover, carbon fibers are low in density, especially compared to steel fibers, their strength-to-density ratio is one of the highest among all fiber types. The main drawback of carbon fibers has been its high cost, and low cost is essential for most applications of cements. During the recent few years, short pitch-based carbon fibers have become common and their prices have steadily dropped. This changed economic picture, together with the attractive properties of carbon fibers, provide impetus to the commercial use of carbon fiber reinforced cements. Such a structure is disclosed in U.S. Pat. No. 4,316,925 dated Feb. 23, 1982 to Delmonte, and in U.S. Pat. No. 4,133,928 dated Jan. 9, 1979 to Riley et al.
In recent years the main development in the field of cements in general has been in the use of additives such as set-accelerating and early strength-enhancing agents and water-reducing agents. Such chemical agents also impart higher compressive strength to the cement. An example of an accelerating admixture is alkaline earth nitrite (i.e., Ca(NO.sub.2).sub.2), 0.01-6 wt. % alkali metal bromide or alkaline earth bromide (i.e., KBr), 0.003-3 wt. % (all based on cement) triethanolamine, together with 15-25 wt. % pozzolan (i.e. fly ash). Examples of water-reducing agents are Na lignosulfonate and polyalkylaryl sulfonate. Although accelerating admixtures have been applied to cements without fiber reinforcements, they have not previously been applied to cements with carbon fiber reinforcements. Without using chemical agents, Akihama et al. required 4 vol. % short carbon fibers in order to double the tensile and flexural strengths, see Concrete International, 10 (1), 40 (1988).
For short random fibers dispersed uniformly in a matrix, the reinforcement effect is relatively low compared with that from aligned continuous fibers. For example, it has been reported that the same flexural strength was achieved with either 3 vol. % short fibers or 0.3-0.5 vol. % continuous fibers. Therefore, even though continuous carbon fibers are more expensive than short carbon fibers, they are used for the reinforcement of cement products such as concrete lids, concrete pipes, etc.
Carbon fibers have an additional advantage of having a high electrical conductivity. Since cement itself is a poor electrical conductor, the presence of carbon fibers greatly increases the electrical conductivity of the cement. The high electrical conductivity makes the cement useful as a material for anti-static flooring, the walling of electromagnetic shield rooms, etc.
The most commonly used carbon fiber length in previous work is 10 mm (0.39 in). However, it has been reported that set cement properties were improved more using 3 mm carbon fibers rather than 10 mm fibers. Furthermore, the molding properties of the cement were deteriorated by the addition of carbon fibers.
For carbon fiber reinforced cements, it is important to have good bonding between the carbon fibers and the cement matrix. For this purpose, organometallic-based coatings, latex coatings, anodic oxidation of carbon fibers, surface treatment with concentrated aqueous HNO.sub.3 solution, surface treatment with chlorosulfonic acid to give hydrophilic carbon fibers and introduction of phosphate groups have been used to enhance the bonding. Water reducing agents and silica fume have been used to maintain good workability in the cement mortar and to make the fibers and the cement matrix contact each other firmly. The above-mentioned methods, though effective, have their drawbacks. The coating and surface treatment of fibers are relatively cumbersome processes. Water reducing agents introduce foam and generate a higher dry shrinkage of the specimen.
Triethanolamine is known to be a setting accelerator, see Ramachandran, Cem. Concr. Res., 6 (5), 623-631 (1976) and forms a complex with the hydrating silicate phase. Sodium sulfate, potassium aluminum sulfate, sodium nitrite and sodium chloride are other ingredients that have separately been used in cements previously, and some of these ingredients have been used together in certain proportions previously, see U.S. Pat. No. 3,891,454 dated June 24, 1975 to Cunningham et al., U.S. Pat. No. 4,058,405 dated Nov. 15, 1977 to Snyder et al., U.S. Pat. No. 3,922,172 dated Nov. 25, 1975 to Crinkelmeyer et al., U.S. Pat. No. 3,801,338 dated Apr. 2, 1974 to Whitaker, and U.S. Pat. No. 4,337,094 dated June 29, 1982 to Tokar.
A concentrated aqueous solution of the sodium salt of a condensed naphthalene sulfonic acid is a surfactant that is known to adhere to reinforcing fibers, preventing the formation of fiber balls and allowing the fibers to be uniformly dispersed in the cement slurry.